1. Field of the Invention
The present invention relates to a power converter control apparatus that is mounted chiefly on a vehicle and controls an AC generator for vehicle having an armature winding and a field winding and operating as a generator after an engine is started.
2. Background Art
A vehicle, such as an automobile, equipped with an AC generator for vehicle (hereinafter, referred to simply as a generator) employs a power converting apparatus for vehicle provided between DC devices, such as a battery and lights, and a generator, which is an AC device, and performing two-way power conversion.
Generally, such a power converting apparatus for vehicle is connected between the generator and the battery and has a multi-phase bridge circuit formed of a plurality of diode elements. The power converting apparatus for vehicle converts AC power generated by the generator to DC power by rectifying the AC power using the multi-phase bridge circuit and supplies the battery and the DC devices with the converted DC power.
Recently, there has been proposed a power converting apparatus for vehicle using a switching element and an accompanying parasitic diode instead of diodes and adopting rectification by the switching element (hereinafter, referred to as synchronous rectification) because efficiency is higher and the element generates less heat when adopting the synchronous rectification than when adopting the rectification using a diode bride. Switching control of the switching element is performed according to a conducting state of the parasitic diode.
An example of the apparatus in the related art is described, for example, in Patent Document 1. According to this apparatus, when the generator starts generating power, diode rectification is performed by the parasitic diode. As an amount of power generation by the generator increases, the diode rectification shifts to the synchronous rectification. The generator generates power in such a manner that temperatures in all the regions of the generator and a generator control apparatus are maintained at or below an allowable temperature. The generator and the generator control apparatus are thus prevented from damage caused by overheat.
In a case where regenerative generation that recovers deceleration energy of a vehicle is performed by the generator control apparatus as above, when the temperatures in all the regions of the generator and the generator control apparatus are not higher than the allowable temperature, an upper limit value of a field current having an influence on an amount of power generation can be set higher during regenerative generation than during normal power generation. Accordingly, because more power can be generated by regenerative generation than by normal power generation, deceleration energy can be regenerated more effectively.    Patent Document 1: JP-A-2010-81741
It should be noted, however, that the generator is used inside an engine room of the vehicle in a hot environment. Hence, in a mode to regenerate deceleration energy, more heat is generated by an armature winding and a switching element in the generator because of a large generated current and the temperatures in the respective regions of the generator and the generator control apparatus rise. It thus becomes necessary to reduce the generated current, which is the cause of heat generation, by lowering the field current so that the temperatures in the respective regions do not exceed the allowable temperature. Patent Document 1, however, neither teaches nor suggests a specific countermeasure.
The temperatures may be lowered by a method of reducing the generated current by lowering the field current, and the field current is lowered by setting a lower upper limit value by multiplying the upper limit value of the field current for every rotation speed determined according to a maximum output characteristic of the generator by a constant coefficient. Alternatively, the temperature may be lowered by a method of reducing the generated current by limiting the field current at a constant value.
According to the method of multiplying the upper limit value of the field current by a constant coefficient, the temperatures are lowered because heat generated by the armature winding and the switching element is suppressed by reducing the generated current by limiting the field current. Accordingly, the upper limit value multiplied by a constant coefficient is the upper limit value of the generated current at a rotation speed with which heat generation reaches a maximum. Hence, the generated current may be suppressed excessively at a rotation speed lower than the rotation speed specified above, because there is a region where heat generation is small due to smallness of the generated current and suppression of the generated current is unnecessary.
At a rotation speed higher than the rotation speed specified above, the generated current is suppressed more than during normal power generation. However, whether a temperature rise is suppressed sufficiently with the suppressed generated current is unknown and therefore whether the temperatures in the respective regions are not higher than the allowable temperature is also unknown.